An assembled battery used for an electric vehicle (EV) and a hybrid vehicle (HV) includes a lot of secondary (i.e., rechargeable) cells connected in series to generate a high voltage ranging from about 100 volts (V) to 400 V. For example, the assembled battery of 300 V is made of 150 lead cells (about 2 V per cell), 250 nickel-hydrogen cells (about 1.2 V per cell), or 80 lithium-ion cells (about 3.6 V per cell) that are connected in series.
The secondary cell, especially, the lithium-ion cell has less resistance to overcharge and overdischarge. If the cell is used outside its specified voltage range, the cell may have significantly reduced cell capacity and produce heat. Therefore, the assembled battery needs cell charge control that keeps voltages of each cell within the specified voltage range. In the assembled battery, each cell has different state of charge, i.e., different voltage, because each cell has different cell capacity and self-discharge characteristic. Therefore, cell voltage equalization is applied to the assembled battery, as disclosed in JP-A-2004-80909 and JP-A-2004-248348.
FIG. 10 is a schematic of a conventional overcharge detection circuit used for an assembled battery 1. The assembled battery 1 includes cells BC1-BC8 connected in series and terminals T1-T8, and TG connected to voltage lines LN1-LN8, and GND, respectively. The terminal TN is a positive terminal of the cell BCN, where N is a positive integer between 1 and 8 inclusive. In other words, the terminal T(M+1) is a negative terminal of the cell BCM, where M is a positive integer between 1 and 7 inclusive. The terminal TG is a negative terminal of the cell BC8. An overcharge detection circuit 2 for the cell BC1 is interposed between the voltage lines LN1, LN2 connected to the terminals T1, T2, respectively. The overcharge detection circuit 2 includes a voltage detection circuit 3 having resistors R1, R2 connected in series, a reference voltage generation circuit 5 having a constant current circuit 4 and a trim resistor R3, and a comparator 6 for comparing a detected voltage with a reference voltage. Each of overcharge detection circuits 2 for the cells BC2-BC8 is similar in structure to the overcharge detection circuit 2 for the cell BC1.
In the structure shown in FIG. 10, each of the overcharge detection circuits 2 for the cells BC1-BC8 needs the constant current circuit 4. Because typically the constant current circuit 4 produces a constant current with a reference voltage generated by a bandgap reference circuit, each of the overcharge detection circuits 2 for the cells BC1-BC8 needs the bandgap reference circuit. Therefore, when the overcharge detection circuits 2 are integrated into a semiconductor integrated circuit (IC), chip size and manufacturing cost of the IC are increased.
A current mirror circuit is widely used in the IC. In the current mirror circuit, output current mirrors input current by a predetermined ratio. The current mirror circuit has a pair of transistors one of which is an input transistor and the other of which is an output transistor. The emitters of the input and output transistors are connected to a power supply line that is grounded. The bases of the input and output transistors are connected to each other. The base and collector of the input transistor are connected to each other directly or through a transistor for supplying base current.
When each of the input and output transistors has the same emitter area and a sufficiently large current gain, the current mirror circuit has a mirror ratio of 1. As a result, the output current becomes equal to the input current. If the current gain is insufficient, the mirror ratio deviates from 1 and the output current becomes smaller than the input current. A current mirror circuit disclosed in JP-A-H9-204232 detects a control current and controls the input current based on the detected control current, thereby reducing the deviation of the mirror ratio.
A resistor may be connected between the power line and a base line to which the bases of the input and output transistors are connected. The resistor reduces impedance of the base line so that resistance to noise can be increased. In a practical circuit, therefore, the resistor is often used to prevent noise. Further, the resistor clamps the electric potential of the base line to the potential of the power line. Therefore, for example, even when system switches to a low power consumption mode and the input current to the current mirror circuit is interrupted, leakage current through the transistors can be prevented by the resistor. As the resistance of the resistor is smaller, the resistor functions more effectively.
However, reduction in resistance of the resistor results in an increase in current flowing the resistor. When the current flowing the resistor becomes equal to or larger than the base current of the transistors, the mirror ration deviates from 1. Therefore, because the resistance of the resistor cannot be reduced below a certain level, the noise and leakage current cannot be fully prevented.